1. Field of the Invention
The present invention relates to a curl correction device for correcting the curl of a curled sheet, and to an image forming apparatus having the curl correction device, such as a copying machine, a printer, a facsimile apparatus or a composite apparatus composed of such apparatuses.
2. Description of Related Art
Image forming apparatuses, such as a copying machine, a printer, a facsimile apparatus, etc., which are arranged to form images on sheets have recently come to be popularly used. These apparatuses are forming images by various methods. Among the known image forming methods, an electrophotographic method whereby a toner image is transferred to and fixed on a sheet is widely employed. The electrophotographic-type image forming apparatus is provided with a thermal fixing device for fixing a toner image to a sheet after the toner image is transferred to the sheet. Further, since energy saving is vary important these days, a two-sided printing function is becoming indispensable to an image forming apparatus.
A laser beam printer of the electrophotographic type is described below, by way of example, as one of the conventional image forming apparatuses having the two-sided printing function, with reference to FIGS. 14 to 17.
As shown in FIG. 14, the conventional image forming apparatus includes, within an image forming apparatus body 1, sheet cassettes 20 each of which is arranged as a sheet housing part to allow sheets P stacked therein, an image forming part 2 which is arranged as an image forming means to form a toner image on a sheet P, and a fixing part 3 which is arranged to fix the toner image formed on the sheet P.
Further, the image forming apparatus is provided with a sheet conveying path 21 which is formed to extend from the sheet cassette 20 up to the image forming part 2, and a two-sided-print conveying path 4 which is arranged, as a sheet reconveying path, to feed the image forming part 2 again with the sheet P, after the sheet P has passed the fixing part 3, for the purpose of printing on both sides of the sheet P. Further, on the downstream side of the fixing part 3, there are provided a sheet delivery path 6 for delivering a sheet P to the outside of the apparatus, and a sheet inverse conveying path 7 for leading the sheet P to the two-sided-print conveying path 4. These conveying paths 6 and 7 are arranged in such a way as to fork away from each other.
Conveying path change-over levers (not shown) are arranged respectively at a fork point between the sheet delivery path 6 and the sheet inverse conveying path 7 and at a confluence point between the sheet inverse conveying path 7 and the two-sided-print conveying path 4. The sheet conveying paths are arranged to be switched from one path over to another by these conveying path change-over levers under control signals from control means (not shown) which is arranged to control the whole image forming apparatus.
Each of the sheet conveying paths is provided with pairs of conveying rollers 22 for conveying the sheet. A pickup roller 23 is arranged to pick up from each of the sheet cassettes 20 a sheet P which is located uppermost among the sheets P stacked at the sheet cassette 20. A pair of registration rollers 24 are arranged to intermittently feed a transfer part 5 of the image forming part 2 with the sheets P at a predetermined timing. A pair of delivery rollers 25 are arranged to discharge and deliver to the outside a sheet P having passed through the fixing part 3. A belt conveyer part 26 is composed of a plurality of belts and a plurality of pulleys which are arranged to feed the fixing part 3 with a sheet P after an image is formed on the sheet P through the image forming part 2.
In the image forming apparatus, a sheet P which is located uppermost among the sheets P stacked at each of the sheet cassettes 20 is picked up by the pickup roller 23. The sheet P picked up is transported along the sheet conveying path 21 by the conveying roller pairs 22 which are disposed after the pickup roller 23. With the sheet P transported in this manner, the registration roller pair 24 intermittently feeds the transfer part 5 of the image forming part 2 with the sheet P. After a toner image is transferred to the upper surface of the sheet P by the transfer part 5, the sheet P is conveyed to the fixing part 3 by means of the belt conveyer part 26.
At the fixing part 3, the sheet P is heated and pressed to have the toner image fixed thereto. In the case of ordinary printing only on one side of the sheet P, the sheet P is discharged and delivered to the outside by the delivery roller pair 25 after the fixing process.
In a case where the apparatus is used for printing on both sides of the sheet P, on the other hand, the sheet P having passed through the fixing part 3 is led by the conveying path change-over lever (not shown) to the sheet inverse conveying path 7 which branches downward after the fixing part 3. While the sheet P is on the movement from the sheet inverse conveying path 7 to the two-sided-print conveying path 4, the moving direction of the sheet P is reversed to move the sheet P further on the two-sided-print conveying path 4 in a state of having its image-formed side facing upward. Then, the sheet P is shifted from the two-sided-print conveying path 4 again to the sheet conveying path 21. On the sheet conveying path 21, the main surface side of the sheet P is inverted to have its image-formed side facing upward. In this state, the sheet P is intermittently conveyed to the transfer part 5 of the image forming part 2. After a toner image is transferred to the upper surface of the sheet P, the sheet P is subjected to the toner image fixing process at the fixing part 3 and is then delivered to the outside by the delivery roller pair 25.
The electrophotographic image forming apparatus has a problem in that the sheet P is curled by heat when the sheet P passes through the fixing part 3. This is called heat curl. After heating by the fixing device 3, the sheet P is curled also while it is passing through a bent part A or B of the conveying path. In a case where the sheet P comes to the two-sided-print conveying path 4, the upper side (image-formed surface) of the sheet P tends to be curled into an upward protruding shape, as shown in FIG. 17. In a case where the image forming apparatus is used for printing on two sides of the sheet P, in order to prevent inadequate image transfer, the curl must be removed before the sheet P is fed to the image forming part 2. To meet this requirement, as shown in FIG. 14, a curl correction mechanism 200 is arranged on the two-sided-print conveying path 4 to correct the protruding curl by retrorsely imparting a curl to the sheet P in the reverse direction.
The curl correction mechanism 200 includes, as shown in FIG. 16, an endless belt 214 arranged to convey the sheet P, stretching shafts 211 and 212 arranged to revolvably stretch the endless belt 214, and a pressing roller 213 which is rotatively arranged to push the stretched endless belt 214 from outside. The curl correction mechanism 200 is thus arranged to convey the sheet P in a state of having the sheet P sandwiched or pinched between the endless belt 214 and the pressing roller 213.
At the curl correction mechanism 200, the sheet P is caused to pass through a path formed approximately in a V shape jointly by the endless belt 214 and the pressing roller 213 of the curl correction mechanism 200 shown in FIGS. 14 and 16. By this arrangement, the upward curl of the sheet P is corrected to prevent inadequate image transfer to the second surface of the sheet P.
The conventional image forming apparatus arranged as described above has presented a problem in the following point.
If the retrorse curl imparted to the sheet P is too strong, the curl correction becomes excessive to bring forth a downward protruding curl. In carrying out the image transfer for the second time through the two-sided-print conveying path 4 under such a condition, a protruding curl arises in such a way as to have the second transfer surface of the sheet P away from the main surface of the transferring photosensitive drum 2a, as shown in FIG. 15. Then, the second transfer tends to be inadequately carried out. To prevent this, the retrorse curling amount to be imparted to the sheet P must be set at an apposite degree at the curl correction mechanism 200. Heretofore, the degree of curl and that of the retrorse curl have been set by using, as a standard, a normal paper sheet (measuring 52 to 90 g/m2 in basis weight).
Meanwhile, in recent years, the number of kinds of sheet materials desired to be useable by image forming apparatuses has increased. Particularly, a desire to use a thick paper sheet measuring 100 g/m2 or more in basis weight (hereinafter referred to as the thick sheet) has become strong.
However, the normal sheet (measuring 52 to 90 g/m2 in basis weight) and the thick sheet (measuring 100 g/m2 and more in basis weight) differ in curling amount even if they are in the same sheet size and even when they are allowed to pass through the same fixing device and the same bent parts of the conveying paths. Hence, two-sided printing on the thick sheet has often become difficult.
In other words, unlike the normal sheet, the thick sheet does not curl upward but rather remains flat or curls somewhat downward on the two-sided-print conveying path 4. It is, therefore, unnecessary to make curl correction by using the curl correction mechanism 200. However, the curl correction mechanism 200 of the conventional image forming apparatus is arranged without taking the use of the thick sheet into consideration but is arranged to make curl correction appositely only to the normal sheet. Hence, when the thick sheet is allowed to pass through the curl correction mechanism 200, the curl correction causes the thick sheet to curl downward too much and thus tends to cause the fore end of the sheet to strongly abut on the main surface of the transferring photosensitive drum 2a. Such a condition has sometimes caused an inadequate image transfer (image deviation) or jamming of paper.
It is conceivable to solve this problem by adjusting the amount of ingression of the pressing roller 213 on the endless belt 214 according to the thickness of the sheet. However, such adjustment necessitates the use of a means for detecting the thickness of the sheet and a means for changing the ingression amount of the pressing roller 213 from one amount to another amount, and thus inevitably increases the cost and size of the image forming apparatus.
Meanwhile, among image forming apparatuses of varied kinds, printers and copying machines which are electrophotographically arranged to be capable of outputting full color images have come to be popularly used. FIG. 18 shows, by way of example, the arrangement of a printer 140 which is of that kind.
As shown in FIG. 18, the printer 140 is arranged to read an original at a reader part 102 provided on the upper part of the printer body 101. Information obtained by the reader part 102 is used to form toner images of yellow, magenta, cyan and black colors on photosensitive drums 5Y, 5M, 5C and 5Bk which are provided for these colors at an image recording part 104. The toner images of these colors are transferred one on top of another to a sheet supplied from a sheet supply part 103. The toner images of the different colors are fixed to the sheet, and, then, the sheet is delivered to a sorter 127, by a fixing-delivery part 106.
Demands have recently increased for arranging an image forming apparatus in combination with some post-processing apparatus, such as a sorter arranged to sort the image-formed sheets by a desired number of sheets, a stapler arranged to staple a sheet bundle, a puncher arranged to punch a hole or holes in the sheet, a finisher arranged to permit sheets to be stacked up in a larger quantity, or the like. Particularly, a demand is becoming high for arranging printers to permit selective use of a back-side sheet delivering function whereby each sheet is delivered in a state of having its image-formed side facing downward.
Compared with an image forming apparatus of the ordinary type, an image forming apparatus having such an post-processing apparatus is more apt to be affected by the size and direction of the curl of a sheet. Excessively curled sheets tend to cause inadequate stacking, inadequate matching, imperfect stapling, etc.
To solve that problem, the printer 140 shown in FIG. 18 has a curled-sheet correction device 126 for correcting a curled sheet disposed on the upstream side of the post-processing apparatus.
The curled-sheet correction device 126 has a curled-sheet correction part 130 which is arranged as shown in FIG. 19. As shown FIG. 19, the curled-sheet correction part 130 includes a sponge roller 128 and a metal roller 129. The sponge roller 128 is made with a sponge layer 128b and a rubber layer 128c formed around a metal core bar 128a. The metal roller 129 is urged in the direction of an arrow F to be pushed somewhat into the sponge roller 128 to form a nip part there.
Each of these rollers 128 and 129 is driven to rotate in the direction of arrows by a drive source (not shown). An upward curled sheet, i.e., a sheet having its two ends curled upward, is corrected to have a horizontal surface by imparting a downward curl to its upward curled ends when the sheet comes into the nip part from the direction of an arrow P.
However, the conventional curled-sheet correction device 126 tends to bring about a slip if a rotative driving force is applied to the metal roller 129, because a frictional force on the sheet is small. The occurrence of such a slip tends to make the sheet conveying speed unstable. If a rotative driving force is applied to the sponge roller 128, on the other hand, the outside diameter of the sponge roller 128 comes to vary to make also the sheet conveying speed unstable, because the sponge part of the sponge roller 128 is deformed by being pushed.
Besides, in a case where the post-processing apparatus, such as a stapler, a puncher, or the like, is disposed on the downstream side of the curled-sheet correction device 126, if the timing of feeding the post-processing apparatus with the sheets, or the conveying speed, is inaccurate, the inaccurate timing might cause jamming or the like to take place on a conveying path located on the downstream side of the curled-sheet correction device 126.
To avoid such a trouble, the conventional curled-sheet correction device 126 is arranged to finely adjust the sheet conveying speed of the curled-sheet correction part 130 or to adjust the sheet conveying speed by arranging an additional pair of rollers on the downstream side. However, such an arrangement makes it necessary to provide an additional control device for controlling the sheet conveying speed. The provision of such an additional control device has caused the curled-sheet correction device 126 to have a complex structure and an increased size.
Further, the curled-sheet correction device 126 has presented another problem in that, in order to make the curled-sheet correction device 126 capable of correcting a larger amount of curl, the amount of pushing the metal roller 129 into the sponge roller 128 must be increased. However, the increased pushing amount would cause the input torque of the above-stated rotative driving force to increase, and thus would cause an increase in cost of the curled-sheet correction device 126.
It is an object of the invention to provide a sheet conveying device which is arranged to be capable of correcting the curl of sheets according to the thickness of the sheets, without incurring any increase in cost and size of the sheet conveying device, and also to provide an image forming apparatus having the sheet conveying device.
It is another object of the invention to provide a curled-sheet correction device simply arranged to be capable of reliably correcting the curl of curled sheets and also to provide an image forming apparatus having the curled-sheet correction device.
To attain the above objects, in accordance with a first aspect of the invention, there is provided a sheet conveying device, comprising an endless belt for conveying a sheet, at least two stretching shafts arranged to revolvably stretch the endless belt, a rotatable pressing roller arranged to push the stretched endless belt from outside, the sheet being arranged to be conveyed in a state of being pinched between the endless belt and the pressing roller, and pressing means for pressing the pressing roller so as to make the pressing roller movable in a direction of being pushed to the endless belt and in a direction of being released from being pushed to the endless belt.
Further, in the sheet conveying device, the pressing means is an elastic member arranged to press a rotation shaft of the pressing roller.
Further, in the sheet conveying device, the elastic member is provided at each of two ends of the rotation shaft of the pressing roller.
Further, in the sheet conveying device, an angle a which upstream-side and downstream-side sheet conveying surfaces of the endless belt formed with the pressing roller taken as a boundary therebetween make with each other is within the following range:
2xcfx80/6(rad)xe2x89xa6xcex1xe2x89xa65xcfx80/6(rad). 
Further, in the sheet conveying device, the pressing roller is located adjacent to an intersection point at which an upstream-side conveying path and a downstream-side conveying path are connected to each other.
Further, in the sheet conveying device, an angle xcex2 which the pressing roller, the endless belt and the stretching shafts make with each other is an angle bending in the same direction as the angle xcex1, and the angle xcex1 and the angle xcex2 are in a relation satisfying the following condition:
xcex2=xcex1xc2x1xcfx80/6. 
Further, an image forming apparatus comprises the above-stated sheet conveying device, and image forming means for forming an image on a sheet conveyed by the sheet conveying device.
In addition, the image forming apparatus further comprises a sheet housing part arranged to allow sheets to be stacked therein, a sheet conveying path formed from the sheet horsing part up to the image forming means, and a sheet resupply path arranged to supply to the image forming means again a sheet having an image formed thereon by the image forming means, wherein the above-stated sheet conveying device is disposed on the sheet resupply path.
Further, in the image forming apparatus, the sheet resupply path has one end on a downstream side thereof connected to the sheet conveying path.
Further, in the image forming apparatus, the sheet resupply path is provided with sheet inverting means for inverting a traveling direction of the sheet having an image formed thereon by the image forming means.
Further, to attain the above objects, in accordance with a second aspect of the invention, there is provided a curled-sheet correction device, comprising an endless belt arranged to circulate in a state of being wrapped around a plurality of guide rollers, and at least one curl correction roller arranged to press the endless belt in such a direction as to narrow a circulating area of the endless belt, wherein, among the plurality of guide rollers, a driving guide roller to which a rotative driving force is imparted is formed to have a straight generatrix, and curl of a curled sheet is corrected while the curled sheet is conveyed jointly by the endless belt and the curl correction roller.
Further, in the curled-sheet correction device, the curl correction roller is arranged to press a tension side of the endless belt.
Further, in the curled-sheet correction device, the curl correction roller is arranged to press a tension side of the endless belt, and the driving guide roller is disposed on a most downstream side in a direction in which the sheet is conveyed.
Further, in the curled-sheet correction device, among the plurality of guide rollers, a driven guide roller arranged to be driven to rotate has a larger diameter at a middle part thereof than at two end parts thereof.
Further, an image forming apparatus comprises image forming means for forming an image on a sheet, and the above-stated curled-sheet correction device arranged to remove curl of the sheet having an image formed thereon by the image forming means.
Further, an image forming apparatus comprises image forming means for forming an image on a sheet, two-sides inverting means for inverting two sides of the sheet having an image formed thereon by the image forming means and then discharging the sheet, and the above-stated curled-sheet correction device disposed at the two-sides inverting means.
The above-stated endless belt is arranged to circulate by the rotation of the guide rollers. In this instance, the endless belt circulates under the pressure of the curl correction roller. Therefore, a part of the endless belt where the endless belt is pressed by the curl correction roller is flexibly bent in the direction of narrowing the area of circulation of the endless belt. The sheet is allowed to pass between the curl correction roller and the bent part of the endless belt. The curl of the sheet is corrected through the flexibly bent part of the endless belt. Besides, since the generatrix of the driving guide roller is arranged to be straight, the revolving speed of the endless belt never fluctuates. The endless belt is thus arranged to be capable of conveying the sheet exactly at a desired speed.
With the curl correction roller arranged to press the tension (stretching) side of the endless belt, the inward pressing (ingression) amount of the curl correction roller on the endless belt little varies even when the ingression of the sheet in between the curl correction roller and the endless belt causes variations in load. Therefore, the endless belt can convey the sheet at an approximately constant speed.
With the curl correction roller arranged to press the stretching (tension) side of the endless belt and the driving guide roller disposed at the most downstream part, the rectilinear recovering tendency of the bent part of the endless belt under the pressure of the curl correction roller is strong. The strong recovering tendency effectively acts to minimize the fluctuations of the inward pressing amount of the curl correction roller on the endless belt even when the ingression of the sheet in between the curl correction roller and the endless belt causes variations in load. Therefore, the sheet conveying speed of the endless belt is approximately constant.
The driven guide roller which is driven to rotate among the plurality of guide rollers is arranged to have the diameter of its middle part set to be larger than the diameters of its two end parts. By virtue of this arrangement, when the endless belt trends toward the end part of the driven guide roller while the endless belt is circulating, the endless belt is prevented from coming off the guide rollers, because the endless belt is then moved toward the middle part, which has a larger diameter.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.